Conventional polymers used in adhesives include vinyl acetate resin-based polymers, chloroprene rubber-based polymers, acrylic resin-based polymers, natural rubber, urethane resin-based polymers, or the like. Chloroprene rubber-based polymers are advantageously used in such applications as solvent-based contact adhesives and graft adhesives because they can have high adhesive strength to a wide variety of adherends in low-pressure bonding. However, regulations on volatile organic compounds (VOCs) and solvents become stricter every year because of the risk of ignition in working environments, the cost required to make special ventilation and recovery facilities for preventing ignition, and recent attention to environmental pollution and human health. To meet these demands, water-based adhesives containing chloroprene rubber-based polymer latex have been aggressively developed so that solvents can be eliminated from adhesives. Unfortunately, adhesive performance requirements are very high in the field of conventional solvent-based adhesive applications, and a problem has been pointed out in that the adhesive strength of water-based adhesives is lower than that of conventional solvent-based adhesives.
Thus, to meet an increasing demand particularly for the improvement of the adhesive strength at the initial drying stage, studies have been made on blending two or more types of polymer latexes so that their disadvantages can be compensated for (see, for example, Patent Literature 1). Unfortunately, since different polymer latexes have different pH regions for stable properties, making a random blend of latexes cannot achieve sufficient initial adhesive strength.
To solve this problem, a device is designed to introduce a chloroprene rubber-based polymer latex and a gelling agent such as a metal salt aqueous solution, as two components, into a coating system without blending them until immediately before coating, and to subject them to physical forced mixing in a spray gun part so that salting-out coating can be achieved (see, for example, Patent Literature 2 and Patent Literature 3). Unfortunately, when this type of two-component coating is performed, not only quality control such as a technique to keep the mixing ratio constant is complicated, but also a trouble with the device, such as clogging of the spray gun, can often occur. Thus, there has been a strong demand for one-component adhesives, and adhesive manufacturers have made various investigations to meet such a demand. Studies have also been made to attain a good balance between adhesive performance and storage stability by using chloroprene rubber-based polymer latex in combination with an amino acid or an inorganic salt (see, for example, Patent Literature 4). Basically, such a technique only attains a balance between tradeoffs and cannot provide a fundamental solution to the problems.
Chloroprene rubber-based polymer latex is highly evaluated because it has a high crystallization rate so that the time required to achieve sufficient adhesive strength can be short. To use chloroprene rubber-based polymer latex in the form of a one-component aqueous adhesive composition, however, chloroprene rubber-based polymer latex needs to be blended with a different type of polymer for complementing the insufficient properties of chloroprene rubber-based polymer latex, such as its insufficient initial adhesion rate.
Concerning conventional techniques on chloroprene rubber-based polymer latex, Patent Literature 1 mentioned above discloses a method of blending an acrylic resin-based polymer latex and a urethane resin-based polymer latex for the purpose of providing contact properties. Unfortunately, since different latexes have different pH regions where they are stable, a simple technique such as simple blending of latexes or addition of a common anionic surfactant and/or a common nonionic surfactant has not yet achieved a one-component aqueous adhesive composition having sufficient storage stability.
There is disclosed a technique for producing a chloroprene rubber-based aqueous adhesive having sufficient strength at an initial drying stage and also having a high level of mechanical stability and suitability for spray, in which polyoxyalkylene alkyl ether sulfate and a pH adjuster are added to the adhesive, and it is also disclosed that when the pH adjuster is used to control the pH of the chloroprene rubber-based polymer latex in the range of 7 to 10, the adhesive can have a good initial strength (see, for example, paragraph[0016] of Patent Literature 5). There is also disclosed a technique for producing a chloroprene rubber-based aqueous adhesive having a good balance among initial adhesion, contact property, storage stability, and spray-coatability, in which an acrylic resin latex containing an acrylic resin emulsion and polyoxyalkylene alkyl ether sulfate as a surfactant are added to the adhesive (see, for example, Patent Literature 6). In these techniques, a surfactant that shows a smaller decrease in water solubility even at a pH of 10 or lower is added to a chloroprene rubber-based polymer to inhibit the degradation of the stability of the chloroprene rubber-based polymer at a pH of 10 or lower. Unfortunately, when such a surfactant is used in a larger amount, the chloroprene rubber-based polymer latex can have too high stability, so that during drying, it can be difficult to achieve gelation of the chloroprene rubber-based polymer latex through an insolubilization phenomenon based on the water solubility of abietic acid used as a surfactant. Thus, the mere use of a surfactant in a chloroprene rubber-based polymer latex cannot achieve sufficient storage stability for practical use of the latex as a one-component aqueous adhesive composition with its initial adhesion unchanged.